One example of the structure of a conventional ladder type filter is shown in FIG. 2. This conventional ladder type filter is constructed using a plurality of piezoelectric resonators utilizing an extension mode of vibration of a square plate. More specifically, a two-stage ladder type filter containing four elements shown in the circuit diagram of FIG. 3 is constructed using rectangular plate-shaped serial resonators 1 and 2 and rectangular plate-shaped parallel resonators and 4.
In FIG. 2, reference numeral 2a denotes an electrode formed on one major surface of the serial resonator 2. The same electrode is formed on the other major surface of the serial resonator 2. In addition, the same electrodes are formed on both major surfaces of the serial resonator 1. On the other hand, electrodes 3a and 4a are respectively formed on the entire surface of both major surfaces of the parallel resonators 3 and 4.
Furthermore, reference numerals 5 to 11 denote metal terminals, which are used for electrically connecting the serial resonators 1 and 2 and the parallel resonators 3 and 4 to each other, in the manner shown in FIG. 3. The metal terminals 5 to 11, together with the serial resonators 1 and 2 and the parallel resonators 3 and 4, are contained in a case 12 made of an insulating material. In addition, an upper opening 12a of the case 12 is closed by a cover member (not shown), to construct one ladder type filter component. In this case, the metal terminals 9 to 11 extended out of the case 12, and are utilized as connecting terminals to the exterior. As shown in FIG. 3, the metal terminals 9, 11 are used as electrodes for the circuit of the conventional ladder type filter.
Meanwhile, when the above described ladder type filter is driven, the serial resonators 1 and 2 and the parallel resonators 3 and 4, which are contained in the case 12 must be vibrated in a desired manner. More specifically, the vibration of each of the resonators 1 to 4 must not be prevented by the case 12 in which the resonators 1 to 4 are contained. Therefore, a so-.called spring terminal having spring properties is used as the metal terminal 11 positioned at an end of the ladder type filter.
In the ladder type filter shown in FIG. 2, a spring terminal is used as the metal terminal 11 so as not to prevent the vibration of the resonators 1 to 4, when the resonators 1-4 are contained in the case 12. Accordingly, additional space is required in the case 12 thereby significantly increasing the overall size of the ladder type filter. For example, the two-stage ladder type filter containing four elements as shown in FIG. 2 measures approximately 7.0 mm by 8.0 mm by 8.0 mm thick when it is constructed as a final ladder type filter component.
Furthermore, in recent years, there has been a demand for a ladder type filter, which in a manner similar to other electronic components is constructed as a surface mounted type electronic component.